Method of producing nodular cast iron



Patented Och 24, 1950 OFFICE' METHOD OF PRODUCING NODULAR CAST IRON 011m Smalley, Larchmont, N. Y.

No Drawing. Application January 17, 1949,

Serial No. 71,387

3 Claims. (Cl. 75-130) This invention relates to cast-iron practice in general, and relates particularly to the production of nodular-graphitic cast iron made by the process of dispersing the carbon in the molten iron by introducing a carbide metastabilizer having in addition a nodular-impelling effect, and thus controlling the structural characteristics to produce a cast iron with the graphite thereof in the nodular form in either a base matrix of pearlite or pearlitic ferrite with improved physical properties and a uniform dense section. Nodular-graphitic cast iron, as the term is used herein, is not cast iron comprising a nodular form of graphite produced by heat treatment as in the malleableizing process.'but cast iron having, the graphite content in virgin-nodular or spherulitic form produced in the casting as the melt changes from a fluid state to a solid state.

' In 1941, applicant disclosed the results of his extensive research ina patent application which has since issued as Letters Patent 2,364,922. This patent discloses the use of tellurium, and other elements of the same group, to influence the combined carbon, and also change the graphite by reducing the length of the graphite flakes even to nodular form, and thus improve the grain structure and mechanical properties.

The first requirement for production of virgin nodular graphitic cast iron is that the majority of the carbon must be in the combined form, or

the adumbrant flake form existing in the original mass of molten iron must be destroyed. Graphite is one of the most refractory substances known, and requires temperatures higher than usually found in ordinary melting practice to take it into complete solution. In average cupola practice it is doubtful whether a metal temperature of more than 2900 F. is ever reached. The maximum amount of carbon, therefore, which, for example, a 2% silicon iron can hold in solution at 2900 F. is around 3.7%. Assuming that the ordinary laws of solution hold, such an iron will dissolve about 3.9% carbon if given sufficient time for solution to take place. Cupola melting, however, is extremely rapid, and the molten iron rarely reaches 2900 F. In consequence, high carbon gray iron melted in ordinary ways always contains some free graphite. This fact is not fully appreciated, but the applicant has proven that, pouring such iron rapidly in cold water and examining it chemically and microscopically, it

does contain minute traces of free carbon.

The effect, then, of coarsely-disposed graphite in the original materials which are charged in the cupola, is to adumbrate in the metal the original flake-graphite structure and reproduce it in the finished casting. If, however, such a graphitic iron is cooled rapidly, as by casting in thin bars or in a chilled mold. the graphite flakes may be broken up into a finely dispersed or granular form. This has been demonstrated on many iron castings from such materials, where the castings are not subject to this rapid cooling, the ordinary, mechanically disposed, graphite-flake structure predominates. By the process under consideration the inventor has in mind the melting of such graphitic irons, but adding a carbide metastabilizer having the additional property of impelling nodularization when followed by the addition of a graphitizi'ng agent.

An object of this invention is a three-step process of-making nodular-graphitic cast iron, comprising as a first step selecting a mixture of raw materials which when melted and then cast will yield a gray iron of combined carbon content under 1.2%, and as a second step introducing into the molten metal a carbide-metastabllizer agent of the type which will disperse or aid in dissolving the carbon, and which has in addition the property of impelling nodularization of later deposited free carbon, and as a third step introducing into the molten metal a graphitizing agent to precipitate the free carbon so that the final structure consists of evenly distributed nodular graphite in a pearlite or pearlitic-ferrite matrix.

Another object of this invention is to melt a ferrous mixture that would normally have a combined carbide content of under 1.2% and a silicon content in excess of 1.5%, and then processing the melt with a carbide-metastabilizer nodularization-impelling agent such as to produce a gray iron with nodular form of graphite.

Another object of this invention is to positively control the addition of the carbide-metastabilizer and the graphitizing agents in relation to the thickness of the castings to be made, and to each other, to produce sound dense castings of nodular-graphite structure in a pearlitic or pearliticferrite matrix on a foundry level of production.

A still further object of this invention is to positively control the addition of the carbidemetastabilizer and graphitizing agents by manipulation of the carbide balance in relation to the thickness of the casting to be made, as indicated by wedge casting tests.

The first step in the process of this invention, instead of starting with a high steel mix and then graphitizing to produce gray iron of fine flakegraphite structure, comprises melting an iron mix which would normally have a combined carbide content of under 1.2% and a silicon content in excess of 1.5% which would normally cast as a gray iron with scattered deposits of saucer-like flake graphite. Then, as a second step, adding a controlled addition of a carbide-metastabilizer agent selected from the group consisting of lithium, calcium, barium, magnesium, strontium,

cadmium. The metastabilizer may consist of a single agent for certain iron melts, and in other instances according to the nature of the iron melt, may be alloyed with such materials as iron,

copper, zinc and manganese to control the carbide stabilizing and nodularization effect upon the molten iron melt. The third step is adding a controlled addition of a graphitizing agent. The control in all steps is related to the thickness of the casting to be made.

This invention contemplates the control of the addition of the carbide metastabilizer in the second step by means of a first wedge test hereinafter referred to as a carbide-effect wedge, and the control of the addition of the graphitizer in the third step by means of a second wedge test hereinafter referred to as a nodular wedge. The wedge test for determining the characteristics of iron is known to those skilled in the art, and is explained in some detail in Patent No. 2,371,654, issued March 20, 1945.

The wedge test comprises the pouring of a casting of a predetermined length and of wedge form in cross section with an acute angle of approximately 20 to 30 degrees. The wedges may be of several sizes, namely, one-half inch base with approximately 28,5" acute angle, threefourths inch base with approximately 26.75 acute angle, one inch base with approximately 25 acute angle, and two inch base with approximately 23.5 acute angle. After the wedge casting is poured and cooled, it is broken in two so that the carbide balance may be observed. Upon observatign, one can discern that the acute angle portion has a whitish appearance while the remaining base portion has a grayish-white appearance. In the white portion, the carbon is generally in combined form, and in the grayishwhite portion the carbon is generally in graphite form. The white portion is unmachinable. The width across the face of the wedge at the zone of demarcation between the white and grayishwhite portions is an indication of the eflect obtained by the carbide metastabilizer.

The knife edge of the wedge test piece cools quickly while the heavy section cools much more slowly, and the result is a varying texture in the casting, from the knife edge to the center, and often a shrink spot in the center. The wedgeshaped casting has an acute angle, defining a knife edge, ranging from approximately 20 to 30 degrees.

Thus, when a mixture is said to have a carbide-effect wedge, it means that the distance across the face of the wedge at the line of demarcation is approximately %2 of an inch and is, in simple terms, referred to as a %2 wedge. Because of the design of the wedges the zone of demarcation will be substantially the same regardless of which size wedge is used. In other words, it clearly portrays the carbide-stabilizing effect produced.

With reference to the first step of the process of this invention, the iron melt which may be used would produce a substantially all-gray wedge with a very small amount of white iron. The zone of demarcation between the gray and white across the fracture of the wedge may be in the neighborhood of only inch. This is the first point of departure from standard practice in that a high steel mix or costly alloys are not a requisite. One advantage of this invention is that under certain market conditions a cheaper charge may be us d in the mix. Another advanteen is that soft graphitic iron may be produced 4 continually from the cupola, and at any time it is desired to have a portion of the melt converted to high strength nodular-graphitic iron as taught by this invention.

The second step, that is, the controlled addition of the carbide-metastabilizer agent, is closely related to the third, or graphitizing step, and to the thickness of the casting to be made. The carbide-metastabilizer agent is added, preferably in the ladle, until a first test wedge, referred to as the carbide-effect wedge," shows a line of demarcation which is longer than five times the length of the wedge value of the untreated iron, but not more than twice the thickness of the casting to be made. For example, if the wedge value for the untreated iron were /32, then the minimum value for the carbide-effect wedge would be War, and if the thickness of the casting to be made were a one-inch casting, then the maximum value for the carbide-effect wedge would be /32, or two times all-white. As a general rule, within limits, the larger the carbide-effect wedge value up to the thickness of the casting to be made, the better will be the carbon distribution and the more complete the nodularization of the graphite.

The nodular wedge value is related directly to the thickness of the casting to be made. After the carbide-metastabilizer agent has been added to produce the correct carbide-effect value, a graphitizer of any suitable type is added producing the nodular wedge line of demarcation. The

nodular wedge should have a maximum value of" /32 times the thickness of the casting to be made for all thicknesses of castings. Therefore, if the thickness of the section to be cast i one inch, then the maximum nodular wedge value would be /32 inch long, which means that the casting will be just white, or almost so. In other words, after the carbon has been taken into solution properly in the second step to the desired white extent, a graphitizer is added as a third step until a wedge test piece will indicate that the wedge line has decreased in length to value less than the thickness of the casting to be made, and the casting will then contain free carbon as nodular graphite. The amount of graphitizer added will determine the length of the line, and the extent of the carbon precipitated. The carbon which is precipitated will be influenced by the nodularization-impelling stabilizer present to become nodular in form.

As the process of graphitization proceeds in the third step to reduce the nodular wedge value from /32 to /32, the casting becomes softer and the quantity of nodular graphit increases. Applicant has determined that a second wedge value of /32 for castings substantially one inch thick; /32 for castings substantially two inches thick, and /32 for castings substantially three inches thick or thicker, is the desirable low limit on the third step.

From the foregoing explanation it will be seen that the greater the amount of the carbide metastabilizer added in the second step, the more carbon will be available to produce nodularization. However, if too much carbide stabilizer is present, its influence cannot properly be overcome by the graphitizing material. Further, the amount of graphitizer added must be accurately controlled to keep the graphitized, or nodular, wedge value such as to produce a sound, dense iron.

The invention has thus far been explained in relation to the control steps employed, but not with regard to the time limitations required. A:

beiore explained, the carbide stabilizer used is of the metastabilizer type, and therefore has a limited functional life being influenced by both temperature and time. This invention contemplates the graphitization step, which is tested by the nodular wedge test, within the time of active life of the metastabilizer.

To illustrate more explicitly gray cast iron melted under ordinary conditions and cast in sand molds contains graphite, in saucer-flakelike form. In order to prevent this, the carbidemetastabilizer agent is added such as to completely dissolve this graphite and to assure that it is all taken into solution in the combined condition after which th graphitizing agent is added to assure the complete nodularization-impelling effect as it passes from the liquid to the solid condition.

The temperature range available to effect this structural change, that is to treat the molten iron with a carbide-metastabilizer agent followed by the graphitiz'ing agent, is extremely limited and is restricted by the temperature of melting and the rate at which these treatments are effected. If the metal is not hot enough, the treatment is incomplete and a, mixed flake and nodular graphite in a defective casting results. It is important, therefore, that the metal be melted sufliciently hot that all of the graphite is taken into solution when the metastabilizer agent is added and that the graphitizing agent be introduced while the effect of the carbid'e-metastabili'zer agent is functioning; and, at the same time, retain sufficient heat for the molten iron to cleanse itself of the chemical reactions involved and permit pouring into clean solid castings.

In the ordinary malleableizing process, this graphitization is done by annealing white iron at temperatures between 1550 and 1700 degrees F. In the three-step process defined herein, the same physical eilect is produced, but while the iron is molten within a limited range of temperature between 15 and 30 per cent superheat above the melting temperature. The object of the control tests, that is the carbon-effect wedge and the nodularization wedge, is to provide a practical instrument to guide the operator in determining both the quantity of carbide-metastabilizer and graphitizing agents to be added within the temperature limits available, so as to produce the structural characteristics and strength properties required.

One skilled in the art of foundry practice will be able to apply the foregoing teaching to produce the desired nodular-graphit structure and the improved physical properties.

This invention has been described as a threestep process having two steps thereof controlled by wedge tests to indicate the time and quantity of addition characteristics. It is to be understood that this invention also embraces the substantially simultaneous addition of the metastabilizer and graphitizer. The invention rests both in the entirely new concept herein explained for converting graphite, and in the control for applying the new concept.

Although the invention has been described in its preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form has been made only by way of example and that numerous changes in the details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit 6 and the scope of the invention as hereinafter claimed.

What is claimed is:

1. The method of producin graphitic cast iron having the free carbon thereof in nodular and spherulitic form comprising, melting a charge of iron of such carbon and silicon content that if cast in the absence of a carbidemetastabilizer agent of the spherulitic nodular impelling type would result in a gray cast iron containing saucer-form flake graphite, and adding said carbide-metastabilizer agent to the melt in quantity sufficient to produce a mottled to all-white iron if cast, the metastabilizer agent dissolving and dispersin the carbon in the melt, and thereafter graphitizing the melt sufficiently to produce a spherulitic nodular-graphite gray cast iron casting from the melt with substantialiy complete absence of saucer-form flake graphite.

2. The method of producing cast iron comprising, melting a mixture of ferrous materials and alloying material of such compositionthat if cast would result in a gray cast iron containing flake graphite, adding a carbide-metastabilizing agent to the melt in such amount to give a carbide-effect wedge indicating that the melt if cast would result in a substantially mottled to allwhite iron, said carbide-eifect wedge value having a minimum value of more than five times the length of the wedge value of the untreated iron,

but not more than twice the thickness of the casting to be made, and graphitizing the molten metal to a wedge value embraced within a range which bears a relationship to the thickness of the castin to be made, said graphitized wedge value being not greater than the section thickness to be cast as a maximum, and said graphitized wedge value having a minimum value of 54;: for casting substantially one inch thick; 94 for castings substantially two inches thick; 7& for castings three inches thick or. thicker.

3. The method of producing cast iron comprising, melting a charge of such composition that if cast in sand would result in a gray cast iron containing flake graphite, adding a carbidemetastabilizing agent to the melt in such amount that the treated melt if cast in sand would result in a substantially mottled to all-white iron, adding a graphitizing agent to the meta-stabilized melt in such amount that the graphitized melt if cast in sand would result in gray iron, and finally pouring a casting from said melt.

' OLIVER SMALLEY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Num er Name Date 2,371,654 Smalley et a1 Mar. 29, 1945 2,467,406 Reese Apr. 19, 1949 2,485,760 Millis et a1. Oct. 25,- 1949 2,488,511 Morrogh Nov. 15, 1949 OTHER REFERENCES Paper No. 875, The Institute of British Foundrymen, 44th Annual Meeting, June 17 to 20, 1947, page 9.

Metals. and Alloys, Sept., 1934, pages 188 and 189.

The Foundry, February, 1936, pages 35 and 86.

The Iron Age, May 20, 1948, page 82. 

1. THE METHOD OF PRODUCING GRAPHITIC CAST IRON HAVING THE FREE CARBON THEREOF IN NODULAR AND SPHERULITIC FORM COMPRISING, MELTING A CHARGE OF IRON OF SUCH CARBON AND SILICON CONTENT THAT IF CAST IN THE ABSENCE OF A CARBINDEMETASTABILIZER AGENT OF THE SPHERULITC NODULARIMPELLING TYPE WOULD RESULT IN A GRAY CAST IRON CONTAINING SAUCER-FORM FLAKE GRAPHITE, AND ADDING SAID CARBID-METASTABILIZER AGENT TO THE MELT IN QUANTITY SUFFICIENT TO PRODUCE A MOTTLED TO ALL-WHITE IRON IF CAST, THE METASTABILIZER AGENT DISSOLVING AND DISPERSING THE CARBON IN THE MELT, AND THEREAFTER GRAPHITIZING THE MELT SUFFICIENTLY TO PRODUCE A SPHERULITIC NODULAR-GRAPHITE GRAY CAST IRON CASTING FROM THE MELT WITH SUBSTANTIALLY COMPLETE ABSENCE OF SAUCER-FORM FLAKE GRAPHITE. 